The invention concerns the refining of liquid heavy hydrocarbon fractions containing in particular sulfur and metal impurities, such as straight-run residues, vacuum residues, heavy crude oils, deasphalted oils, pitch and asphalts in admixture with an aromatic distillate or coal hydrogenates.
The problem arising in refining these fractions is to convert them to lighter cuts, free of hetero-atomic impurities and having an improved hydrogen to carbon (H/C) ratio, suitable for use as basis material for the production of fuels such as fuel oils, gas oil and gasoline. For this purpose, it has already been proposed to use hydrotreatment processes, the general term of hydrotreatment being used to define processes involving chemical reactions of hydrogen with the fractions to be refined, in the presence of one or more catalysts, at temperatures generally from 250.degree. to 450.degree. C. and under pressures generally from 0.1 to 30 MPa.
However, in the present state of the art, it is not possible to treat economically charges containing amounts even as low as 10 parts per million of parts of metal impurities such as nickel, vanadium, iron, titanium, etc. . . As a matter of fact, these impurities precipitate within the pores of the hydrotreatment catalysts and are one of the main causes of a quick and irreversible deactivation of these catalysts, which generally withstand the accumulation of at most a few grams of metals per hundred grams in their pores before losing their activity, particularly their desulfurizing activity necessary to fulfill the required product specifications. The cycle periods are then very short, thus leading to a substantial decrease in the productivity of the plant, and the catalyst consumptions are high, thus leading to prohibitive operating costs.
However, a substantial improvement has been recently obtained (French patent application No 82/10757 of June 17, 1982) by using catalysts of high demetallation activity associated with to a high resistance to poisons. These catalysts also have other useful properties, particularly a substantial desulfuration activity.